Guardians of the Stars

Gliese 876

Gliese 876 is a red dwarf approximately 15 light-years away from Earth in the constellation of Aquarius. It was the third closest known star to the Sun confirmed to possess a planetary system (after Gliese 674 and Epsilon Eridani ) and the closest such system known to consist of multiple planets. As of 2011, four extrasolar planets have been found to orbit the star.

The planetary system is also notable for the orbital properties of its planets. It is the only known system of orbital companions to exhibit a triple conjunction in the rare phenomenon of Laplace resonance (a type of resonance first noted in Jupiter’s inner three Galilean moons). It is also the first extrasolar system around a normal star with measured coplanarity.

As a red dwarf, Gliese 876 is much less massive than the Sun: estimates suggest it has only 32% of the mass of the Sun. The surface temperature of Gliese 876 is cooler than the Sun and the star has a smaller radius. These factors combine to make the star only 1.24% as luminous as the Sun, and most of this is at infrared wavelengths.

Initial estimates of the age and metallicity was difficult due to the formation of diatomic molecules in their atmospheres, which made the spectrum extremely complex. By fitting the observed spectrum to model spectra, it is estimated that Gliese 876 had a slightly lower abundance of heavy elements compared to the Sun (around 75% the solar abundance of iron). Based on chromospheric activity the star is likely to be around 6.5 to 9.9 billion years old, depending on the theoretical model used. However, the low rotational period of the star as well as its membership among the young disk population suggest that the star is between 0.1–5 billion years old.

Like many low-mass stars, Gliese 876 is a variable star. Its variable star designation is IL Aquarii and it is classified as a BY Draconis variable. Its brightness fluctuates by around 0.04 magnitudes. This type of variability is thought to be caused by large starspots moving in and out of view as the star rotates. Gliese 876 emits X-rays.

Gliese 876 has a notable orbital arrangement. It was the first planetary system around a normal star to have mutual inclination between planets measured without transits. Later measurements reduced the value of the mutual inclination. The system has the second known example of a Laplace resonance with a 1:2:4 resonance of its planets. The first known example was Jupiter’s closest Galilean moons – Ganymede, Europa and Io. Numerical integration indicates that the coplanar, four-planet system is stable for at least another billion years. This planetary system comes close to a triple conjunction between the three outer planets once per orbit of the outermost planet.

The outermost three of the known planets likely formed further away from the star, and migrated inward. Two of the middle planets are located in the system’s habitable zone, however they are gas giants.

The Ares Corporation sent a mission to the system after the UEG exploration ship returned to Earth after having done a complete system survey. The UEG were not terribly interested in it because it held no habitable systems, however it did have massive resources and Ares was interested in those. The Ares company decided to get all the benefits of being a colony until the UEG started their high taxation of the colonies, then they joined in with the other rebel colonies.